Powder spray systems are well known in the art and representative systems are disclosed, for example in U.S. Pat. No. 5,261,934 ('934), assigned to Nordson Corp. of Westlake, Ohio, the assignee of this invention, which patent is incorporated in its entirety herein. As disclosed in the patent '934, the practice of powder coating involves spraying a powdered coating material onto an object or workpiece and thereafter heating the object and the powder so that the powder melts. When subsequently cooled, the melted powder forms a solid, continuous coating on the object. In many instances, an electrostatic charge is applied to the sprayed powder and the object is electrically grounded to increase the quantity of powder which attaches to the object and to assist in retaining the powder thereon.
Powder deposition is usually performed in a spray booth, i.e., an enclosure wherein any oversprayed powder which is not deposited on the object can be collected. Conventionally, the containment of oversprayed powder in the spray booth is aided by an exhaust system which creates a negative pressure within the spray booth and draws the powder entrained in a stream of air out of the spray booth into a powder recovery unit, also called a powder collector. In the powder collector, the particles of powder are separated from the air by a filter media, collected in a hopper, and then usually returned to the powder supply for sieving and recirculating to the spray gun. The resulting cleaned air, now free of the powder, is usually passed through final filters and discharged into the room or recirculated back to the conditioned air supply for the spray booth.
One problem associated with make-up spray booths of the type described above is to obtain a constant velocity air flow regardless of the variation in the available volume for air flow through the interior of the booth, while concurrently ensuring that contaminants do not enter or leave the booth interior. This type of constant velocity air flow within the spray booth is desirable to minimize disruption of the flow path between the powder dispensing devices and the object so that a uniform coating is obtained on the object and to further ensure that a high percentage of the amount of powder being sprayed attaches to the object, i.e., a high transfer efficiency.
It has been suggested that one way of obtaining such constant velocity flow conditions within the spray booth is to operate an air infeed blower and an air exhaust fan at a relative speed with respect to each other so that the quantity of air entering the booth is equal to the quantity of air withdrawn from the booth. A problem with this design is that no provision is made for accommodating changing conditions within the booth interior caused by the number and position of the objects moving therethrough. That is, no accommodation is made for variations in the volume of air required to maintain constant downdraft velocity in the spray booth as the progression of a object through the application area displaces actual make-up air volume requirements.
These limitations have been addressed to some extent in systems of the type disclosed in U.S. Pat. No. 4,653,387 to Osawa et al. This patent discloses air feed-type paint spray booths in which the air flow through the booth is varied in accordance with sensed conditions in the booth interior such as booth pressure and/or the air velocity at the inlet and outlet of the booth. For example, the Osawa patent includes an air infeed fan and an air exhaust fan whose speed of operation are varied in response to the air flow sensed at the inlet and/or outlet to the booth. One problem with this design is the inaccuracy of the pressure and/or velocity measurements taken within the interior of the spray booth, particularly when coating physically large objects which displace substantial quantities of air in the course of movement through the booth, i.e., as the object enters or exits the booth compared to intervals when no object is present at the booth inlet or outlet. Another problem is that the air velocity can be so low in the booth interior that it is difficult to measure, which in turn can result in the velocity of the air moving through the booth being incorrectly adjusted because the speed of the air infeed blower and/or air exhaust fan is dependent upon such measurements of velocity.
Another type of make-up type spray booth is disclosed in U.S. Pat. No. 5,095,811 ('811) to Shutic et al., assigned to Nordson Corporation, the assignee of the present invention, which patent is incorporated in its entirety herein. In the design of the '811 patent, the interior of the spray booth is divided into separate coating zones divided by transition zones. The air infeed and exhaust devices associated with each coating zone and each transition zone are operated to vary flow rate within the booth interior in the course of movement of the item passing through such that the air velocity in each coating zone is maintained below a predetermined maximum downdraft velocity throughout the coating operation, and such that a slightly negative pressure is maintained within the booth interior.
While the previously discussed booth designs have partially addressed the problem of providing a constant air flow through a spray booth, there are still deficiencies in these booth designs relating to the control of the air flow through the powder coating booth to offset the change in air flow corresponding to the build-up of coating powder on the exterior surface of the cartridge filters in the powder collector from the passage of the air entrained stream of oversprayed powder being drawn from the booth into the powder collector. That is, the amount of air which can be drawn through the filters is directly dependent upon the build up of powder on the cartridge filter. This variation in air flow causes problems both at start up when the filters are clean and a high volume of air can be drawn the filters and then after some period of time when the powder buildup allows a much lower volume of air to be drawn therethrough. Also, the filters are periodically subjected to pulse cleaning when a blast of air, directed through the inside of the filter, causes the accumulated powder on the outside surface of the filter to fall off. This, will cause a rapid rise in the volume of air flow through the filter and often change the air flow within the powder booth. As previously mentioned, changes in the air flow through the booth can reduce the transfer efficiency and change the spray pattern from the spray guns.
This problem is partially resolved by seasoning the cartridge filters, that is spraying the cartridge filters with powder prior to coating parts with powder, so that the air flow is initially reduced. Cartridge filters can require from 3 to 30 pounds of powder during seasoning. While the variation in the air flow through the spray booth is reduced, seasoning is a time consuming, expensive operation. The reduction in variations in air flow within the powder coating booth also lessens the effect on the spray patterns emitted from the spray gun. Further, it is desirable to improve the transfer efficiency of the powder coating material onto the parts being sprayed by closely controlling the speed of the fan drawing the oversprayed powder into the powder collector from the powder booth.
Another problem relating to the prior art booth designs is that the motor operated fans, used to draw the stream of air entrained, oversprayed powder from the booth into the powder collector, use a great deal of power and generate a high level of noise.